This invention relates generally to process control systems, and more particularly to an override process control system in which a selector station operates in conjunction with two or more electronic controllers, only one of which governs a final control element, the other controllers being on a standby basis.
An electronic controller is a component in a process control loop that is subject to disturbances, the controller acting in conjunction with other devices to maintain a process variable at a desired value. The factor controlled may be flow rate, pressure, viscosity, liquid level, or any other process variable. In operation, the electronic controller receives, in terms of corresponding input signals, both the process variable and a set point, and it compares these electrical values to produce an output signal that reflects the deviation of the process variable from the set point. This output signal, when applied to a final control element, will directly or indirectly govern the process variable.
Thus one input signal to a controller may be derived from a flowmeter whose reading is converted into a corresponding electrical value, and the output signal may be impressed on a flow-regulating valve which is caused to assume an intermediate position between open and closed at which the flow rate conforms to the set point. The set point generator may be an internal component of the controller or a remotely-controlled device. A typical electronic controller is that manufactured by Fischer & Porter Co. O Warminster, Pa., and described in their Instruction Bulletin (1974) for the Series 53 EL 4000 Electronic Controller. The disclosure of this bulletin is incorporated herein by reference.
Variations in controller action are obtained by adjustment of parameters associated with the control modes and are available in several combinations. These modes of control action which are combined to adjust the controller output signal are known as proportional, reset and derivative.
Proportional action produces an output signal proportional to the deviation of the controlled process variable from the set point. The amount of deviation in terms of percentage required to move the final control element through the full range is known as the proportional band. Automatic reset action, also known as integral action, produces a corrective signal proportional to the length of time the controlled variable has been away from the set point, while derivative action, also known as rate action, produces a corrective signal proportional to the rate at which the controlled variable is changing. Manual reset action is an operator-actuated potentiometer controlled to produce a corrective signal directly proportional to the magnitude of the adjustment.
"Slideback" action permits the transfer from manual to automatic control without a sudden change in output signal. This means that the operator can switch the controller from manual to automatic without an instantaneous jump even when there is a difference between set point and process.
There are some practical situations in which it becomes essential to provide an override control system to regulate a process with only a single final control element from two or more process variables that are interdependent and which must not exceed certain maximum and/or minimum safe limits.
In a situation of this type, control of the process requires that the system always control from the variable that has the greater tendency at any instant to depart from the control point in the undesired direction. In this system, two or more process variables are related in such a way that either can be controlled by the same manipulated variable. An override control system for this purpose requires an override selector station associated with several electronic controllers respectively responsive to the interdependent process variables.
One known type of selector station for an override process control system is that manufactured by Fischer & Porter Co. and described in their Instruction Bulletin (1972) for Series 53 EL 3090-B Override Selector Station. Another known type is the SPEC 200 Automatic Selector Control Systems manufactured by Foxboro Corporation and described in their Technical Information TI 200-225, published September 1973.
In an override control system of the type heretofore known, the final control element is manipulated by the selector station to prevent two or more process variables from exceeding specified limits. Each process variable is fed to a respective electronic controller included in the override system, the set point of the controller representing a limiting safe operating point (maximum or minimum) for that variable. All controller outputs are fed to the selector station which transmits the highest or lowest controller output as determined by a high-low switch on the selector, to the final control element.
When the process variable of one or more of the controllers approaches an unsafe condition, the conroller output will start to change to an extent determined by how unsafe the variable has become, as determined by the deviation from the set point and by the controller mode settings. At any given time, the process variable which is most unsafe is that producing the greatest output change tendency. And since this variable will produce in its associated controller the highest (or lowest) output, it will be chosen by the selector station to control the final operator.
To illustrate the operation of a typical system employing an override selector station of the known type, we shall assume an arrangement in which fluid is drawn into a compressor and discharged thereby into a process through a control valve (final control element). In order to avoid cavitation in a turbine pump, one must prevent loss of suction pressure in the event the discharge pressure is also low. To this end, a sensor on the suction side of the compressor transmits a process variable input signal to a "suction" electronic controller, while a sensor on the discharge side of the compressor transmits a process variable input signal to a "discharge pressure" electronic controller.
The output of the "suction" controller as well as that of the "discharge pressure" controller is applied to the override selector station whose output signal acts to govern the single final control element valve in the compressor discharge line. Normally, the output of the "discharge pressure" controller is selected by the override station to adjust the output pressure of the compressor. But if the suction pressure drops below the set point of the "suction" controller, the override station will then select this controller to take over control of the valve, in which case the "discharge pressure" controller is inactive.
In all override control systems of the type heretofore known, the function of the override station is to select the output signal from one of several process controllers, and to generate an equivalent output signal which is applied to the single final control element. A voltage signal proportional to the selected output signal is transmitted to each of the several electronic controllers which make up the system. In the then selected controller, this voltage signal is used as "feedback," whereas in the non-selected controllers, the voltage signal serves as a reset update to prevent these controllers from going into "Reset Wind Up."
Thus existing types of override selectors require an output line from each controller to the selector station and a second return connection for the feedback and reset update to each controller internal-feedback terminal. If, therefore, it were decided to convert a process control system having a group of electronic controllers to operate in conjunction with an override selector station of the type heretofore known, major wiring changes would be required, with a resulting prolonged down time.
Also while existing forms of electronic controllers are capable of switching over from automatic to manual operation, when such controllers are associated with an override selector station of the type previously known, one cannot effect such transfer by switch-over means already included in the controller. The reason this cannot be done is that in the conventional override system, the final control element is operated not by one of the controllers but by an output signal generated in the override selector station.
It is necessary, therefore, either to incorporate a manual-automatic transfer means in the override station or to provide a separate manual/automatic transfer station in the output line, further adding to the cost of the override installation.